Polyimides are currently used to a large extent in microcircuit fabrication as carrier layers and insulating layers in electronic packaging of semiconductor chips and micro-circuitry. When the polyimides are provided on a substrate, such are normally applied as precursors in the polyamic acid form that contains free carboxylic acid groups. For the most part, the fabrication of photopatterned polyimide layers is carried out using non-photoreactive polyimide precursors with the aid of conventional photoresist materials. In addition, there have been suggestions to directly photopattern photocrosslinkable polyimide precursors to thereby significantly reduce the number of processing steps. For example, see U.S. Pat. Nos. 4,670,535 and 4,778,859.
There are a number of commercially available photocrosslinkable polyimide precursors including those based on the chemical principle generally referred to as "Siemans Technology." For instance, one typical photosensitive polyimide precursor is a methacrylate polyimide ester. Also, such formulations typically include a multifunctional monomer along with the photosensitive polyimide precursor such as pentaerythritol triacrylate, trimethylolpropane triacrylate and tripolyethylene oxide triacrylate. It has been found that crosslinking through multifunctional monomers decreases solubility-in-developer faster, which in turn tends to increase photospeed and resolution in negative photoresist formulations. However, these types of formulations undergo solvent swelling during development, which, in turn, decreases resolution and less of thickness and adhesion upon development.
It has also been suggested to provide photosensitivity in a polyimide through the ionic salt complexation between a tertiary amine acrylate, such as dimethylamino-propanol methacrylate, and the carboxylic acid functionality of the polyamic acid. These formulations are much easier to synthesize than the polyester imides. However, such formulations exhibit low photocontrast, resolution, and sensitivity. Also, such require water-solvent developer mixtures and relatively long development time in the vicinity of about six minutes.
Accordingly, it would be desirable to provide a photosensitive composition of a polyimide precursor that exhibits increased resolution and sensitivity along with reduced swelling and thickness loss upon development. It would also be advantageous to limit the adhesion loss upon development.